Transition State Model for Grain Boundary Motion During Ion Bombardment
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TRANSITION STATE MODEL FOR GRAIN BOUNDARY MOTION DURING ION BOMBARDMENT 6 HARRY A. ATWATER(M), CARL V. THOMPSONM ), AND HENRY I. SMITH(c) Massachusetts Institute of Technology, Cambridge, MA 02139
ABSTRACT Ion bombardment of polycrystalline Ge, Si, and Au films leads to rates of grain boundary motion that greatly exceed rates of thermally-induced motion at the same temperature and which exhibit a weak temperature dependence. The enhanced migration rate is proportional to the rate of energy deposition in nuclear collisions at or very near the grain boundary. Experimental work is reviewed, and a transition state model is presented which accounts for the observed kinetics of grain boundary migration during bombardment. This model suggests that the rate limiting step in grain boundary motion may be thermally-induced migration of a bombardment-generated defect across the boundary. Also, the ratio of atomic jumps at grain boundaries to the local collision-induced Frenkel defect generation rate is shown to be characteristic of each material, but independent of ion mass and ion flux. The model is extended to the motion of an interface between two phases, and applications to crystallization during ion bombardment are discussed. INTRODUCTION Enhancement of the rates of atomic and interfacial rearrangement during ion bombardment has been the subject of several recent experimental studies[I-71. Although models have been developed for thermally induced motion of crystal-crystal and amorphouscrystal interfaces[8-10], less progress has been made in describing interface motion during ion bombardment. A key point to consider in the description of bombardmentenhanced kinetic processes in solids is the determination of whether the kinetic enhancement is limited by migration of beam-generated Frenkel defects, or by a direct elastic or inelastic collisional process at the interface. Investigations of bombardment-enhanced epitaxial crystallization of Si have yielded models for motion of the amorphous-crystalline interface based on both mobile Frenkel defects[t] and also direct interfacial interactions[2]. Recently, studies of bombardment-enhanced grain growth in Ge[3,4] and Ni[5,6] have shown that grain boundary motion is proportional to the energy deposited in elastic collisions in the solid. Here we propose that grain boundary motion is due to bombardment-induced defect production at the interface, and we develop a transition state model to describe this interaction. Comparison is made with experimental results for grain boundary motion in thin polycrystalline films of Ge, Si and Au[71. EXPERIMENTAL RESULTS Grain growth during ion bombardment was first noted in self-implanted Cu samples[1I], and was also seen in ion mixing studies using Au and Ni[12]. Systematic studies of ion bombardment enhanced grain growth have recently been carried out using Ni[5,6,13],
Mat. Res. Soc. Symp. Proc. Vol. 100.
1988 Materials Research Society
346
5 X1013/cm2
No I/I
M
5 X10" /cm2
(C.,)
1 Pm
(b)
1 XlO1O/cm2
(d)
5
6
7
8
Log grain size
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